KR20000016037A - High voltage ac apparatus - Google Patents

Abstract

PURPOSE: A high voltage AC apparatus is appropriate to connect directly to an electric power supply or electric power transmission network and prevents the defect and the cracking generated due to the thermal movement at a winding. CONSTITUTION: The high voltage AC apparatus to be connected directly to a electric power supply or electric power transmission network(16) includes at least more than one winding. The above winding includes at least one current carrying conductor, a first layer having the semiconductor characteristics provided on the whole circumference of the conductor, a solid insulator provided on the circumference of the first layer and a second layer having the semiconductor characteristics provided on the circumference of the insulator. And also, grounding means(18, 24, 26, 28) are provided to connect to the ground at least at one point of the winding.

Description

High Voltage AC Device

A generator with a rated voltage of up to 36 kv fouled October 15, 1932. "36 kV Generators Arise from Insulation Research" by Paul R. Siedler, pp. 524-527. These generators include windings formed of medium voltage insulated conductors and the insulation is divided into multiple layers of different dielectric constants. The insulating material used is formed from various combinations of three components: micafolium-mica, varnish and paper.

In the April 1984 publication by the Power Research Institute EPRI, EL-3391, the generator concept proposed that the generator provide a high voltage so that it could be directly connected to the power network without any intermediate transformers. The generator is assumed to include a superconducting rotor. The magnetization capacity of the superconducting field allows the use of air gap windings of sufficient thickness to withstand electrical forces. The proposed rotor, however, is a complex structure with very thick insulation which greatly increases the size of the device. In addition, specific methods must be taken to insulate and cool the coil end sections.

By high voltage AC device is meant, according to the invention, a rotary electric device, such as a generator of a power plant for power generation, a dual feeder device, an external pole device, a synchronization device, an asynchronous converter cascade, and a power transformer. In order to omit the transformer and connect it to a distribution and transmission network, commonly referred to as a power network, the transformer needed to transform the voltage up to the network level, ie in the range of 130 to 400 kV.

By manufacturing the windings of these devices consisting of insulated high voltage conductors with solid insulation with a structure similar to the cables used for power transmission, the voltage of the device increases to a level such that the device can be directly connected to any power network without an intermediate transformer. Can be. Thus the transformer can be omitted. Typical operating range for these devices is 30-800 kV.

This type of device requires special attention to grounding problems.

Grounding of the generator system and other similar electrical systems uses a planned method for connecting the electrical system to ground potential. When the so-called neutral point of the system is used, the neutral point is often connected to ground directly or through a suitable impedance. The other point of the system is also connected to ground. If one point of the system is grounded, the entire system is grounded as long as the galvanic connection is extended.

The grounding principle used is determined by the design of the system. The following grounding method is most common for systems that include a generator connected directly to a Y-Δ connected incremental transformer with a Δ winding at the generator voltage.

High resistance ground

-No grounding

Resonant ground

High resistance grounding is realized by connecting the primary winding of the distribution transformer connected from the generator neutral point to ground and the low ohm resistor at the secondary side of the distribution transformer. This prior art ground is shown in FIG. 1, which shows a generator 2 connected to the network 9 by a Y-Δ connected incremental transformer 3. The primary side 11 of the distribution transformer is connected between the neutral point of the generator 2 and the ground. The resistor 12 is connected to the secondary side 10 of the transformer.

The same kind of ground can be obtained by installing a high ohmic resistor directly between the generator neutral and ground.

An ungrounded electrical system has no planned connection to ground. So an ungrounded generator has no connection between the neutral and ground except the voltage transformer to provide to other relays and appliances.

Resonant ground is realized as shown in FIG. 1 with a resistor 12 replaced by reactor 12a. The reactance of the reactor is chosen such that during line-to-ground faults the capacitive current is neutralized by the same component of the induced current provided by reactor 12a.

Small resistance or small impedance grounding and effective grounding of the system are possible. Small resistance or small impedance grounding results in smaller transient overvoltages but higher ground fault currents that can cause internal damage to the device.

Small resistance grounding is achieved by deliberately inserting a resistor between the generator neutral and ground. The resistor can be inserted directly into the ground when connected or indirectly into the secondary side of the transformer, where the primary side is connected, for example, between the generator neutral and ground of FIG. 1.

Small impedance ground, a small inductance ground, is achieved by replacing the inductor for a resistor in the same way. At ohms the value of the inductor is smaller than required for resonant ground as described above.

Small resistance or small impedance grounds are suitable for systems containing several generators connected to a common supply line or bus with a circuit breaker between the generator terminals and the common bus.

In order to effectively ground, the generator's neutral point has the same advantages and disadvantages as small resistance or small impedance grounds with some differences.

The system is effectively grounded if any impedance requirement is met that limits the ground impedance magnitude. In an effectively grounded system, the maximum phase-to-ground voltage in the fault-free phase is limited to 80% of the phase-to-phase voltage in the case of ground faults.

The power system network is mainly grounded through the neutral ground connection of the transformer in the system and is so directly grounded or impedance-free (except for contact resistance).

Known grounding techniques are described in IEEE C62.92-1989 of the IEEE Guide for Neutral Grounding Applications in Ground-II Partial Electrical Uniform Systems of Synchronized Systems, published by the Institute of Electrical and Electronics Engineers, New York, September 1989. Are described.

If the generator neutral point is grounded through a small resistance or inductance as described above, a third harmonic current is formed in the path from the generator neutral point to ground. If a directly grounded or small impedance grounded transformer winding or other small impedance grounded generator is directly connected to the generator, the third harmonic current will cycle between them under normal conditions.

Techniques for solving problems in the third harmonic current of a generator and motor operation of an AC device according to the present invention are described in Swedish patent applications 9602078-9 and 9700347-9.

The present invention relates to a high voltage AC device connected directly to a distribution or transmission network and comprising at least one winding.

1 shows a prior art grounding of a synchronous generator.

2 shows an embodiment of an insulated conductor used in the winding of the device according to the invention.

3 shows an ungrounded three-phase device in the form of a Y-connected generator or motor connected to a power system.

4-13 shows another grounding embodiment of the Y-connector of FIG.

14 shows an apparatus according to the invention in the form of a Δ connected generator or motor connected to a power system.

FIG. 15 illustrates the use of a ground transformer in the system shown in FIG. 14.

It is an object of the present invention to provide a high voltage AC device suitable for direct connection to a power distribution or transmission network as described above, wherein the device is provided with grounding means suitable for other uses and operating conditions of the device.

This object is achieved using a high voltage AC device of the kind defined in the introductory part of the description and having the features of claim 1.

An important advantage according to the invention is that the electric field is almost zero at the end region of the outer winding of the second layer having semiconductor properties. Thus, the electric field does not need to be controlled outside the windings, so that electric field concentrations cannot be formed in the sheet, the winding end region, and the changing region between them.

According to a preferred embodiment of the device of the invention, at least two adjacent layers have substantially the same coefficient of thermal expansion. In this way defects, breaks, etc. resulting from thermal motion in the windings are prevented.

According to another preferred embodiment of the device of the invention, the grounding means comprises means for low resistance grounding to the winding. In this way, transient overvoltage and ground fault currents can be limited to suitable values.

According to another preferred embodiment of the device of the present invention, the device has a Y connecting winding, the neutral point of the Y connecting winding is available, and the high resistance grounding means has a secondary side of the transformer whose primary side is connected between the neutral point and ground. Contains the register connected to. In this way the resistors used on the secondary side of the transformer have a relatively low ohms value and a robust construction. Sufficient damping to reduce the transient overvoltage to a safe level is achieved with a resistor of the appropriate size. In addition, mechanical stress and fault damage are limited by the limitation of fault current during line-to-ground faults. The grounding device is more economical than the direct insertion of a high ohmic resistor between the generator neutral and ground.

According to another preferred embodiment of the device of the invention, the device has a Y connection winding and the neutral point of the Y connection winding is available and the grounding means is connected to the secondary side of the transformer whose primary side is connected between the neutral point and ground. And a reactor having the property of causing the capacitive current to be substantially neutralized by the same component of the induced current provided by the reactor during the ground fault. In this way, the net fault current is reduced to a small value by the parallel resonant circuit, which is essentially in phase with the fault voltage. The voltage recovery on the fault phase is very slow in this case and any ground fault of transient nature is automatically eliminated in the resonant ground system.

According to another preferred embodiment of the device of the invention, the grounding means comprises a Y-Δ ground transformer or a so-called zigzag ground transformer connected to the network side of the device. The use of such a ground transformer is equal to low inductance or low resistance ground in terms of fault current level and transient overvoltage.

In order to explain the invention in a more detailed embodiment of the device according to the invention, what has been chosen as an embodiment will be described in more detail with reference to the accompanying drawings, Figures 2-11.

2 shows an insulated conductor that can be used in the windings of the device according to the invention. The insulated conductor comprises at least one conductor 4 composed of a plurality of non-insulated and insulated strands 5. Around the conductor 4 is provided an inner semiconductor layer 6 in contact with at least some of the non-insulated strands 5. These semiconductor layers 6 are in turn surrounded by cable main insulation in the form of a protruding solid insulating layer 7. The insulating layer is surrounded by the semiconductor layer 8. The conductor area of the cable is between 80 and 3000 mm 2 and the outer diameter of the cable is between 20 and 250 mm.

3 schematically shows an ungrounded high voltage AC device in the form of a Y connected generator or motor 14 connected directly to the power system 16.

4 shows grounding means in the form of an overvoltage protector, such as a nonlinear resistance arrester 18 connected between the neutral point 20 of the Y-connector 14 and ground. The nonlinear resistance arrester 18, connected to a neutral point, protects the insulated conductors used in the mechanical winding against transient overvoltages such as overvoltages caused by lightning strikes.

5 shows an embodiment having a high ohm resistor 22 connected in parallel to a nonlinear resistance arrester 18. The nonlinear resistance arrester 18 functions in the same manner as the embodiment shown in FIG. 4 and sensitively detects a ground fault by measuring the voltage across the resistor 22 using the resistor 22.

6 illustrates an embodiment having a high resistance ground of neutral point 20. In this embodiment, a technique similar to the prior art described in connection with FIG. 1 is used. The resistor 24 is thus connected to the secondary side 26 of the transformer having the primary side winding 28 of the transformer connected to ground from the neutral point 20 of the device 14. The resistor 24 is of relatively low ohms and robust construction compared to the high ohm resistors needed for a direct connection between the neutral point 20 and ground to achieve the same result. The voltage rating of the resistor can be reduced as a result. Also in this case the nonlinear resistor arrester 18 is connected in parallel to the primary winding 28. In this embodiment mechanical stress and defect damage are limited during line to ground faults by limiting the fault current.

The resonant grounding of the device can be realized in the same way by replacing the resistor 24 with a reactor having the characteristic that the capacitive current during the line-to-ground fault is neutralized by the same component of the induced current provided by the reactor. Thus the net fault current is formed to be reduced by the parallel resonant circuit and the current will be essentially in phase with the fault voltage. After eliminating the fault, the voltage recovery on the fault phase is very slow and the recovery is determined by the ratio of the effective resistance to inductive reactance of the transformer / reactor coupling. Thus any ground fault of transient nature will automatically disappear in the resonant ground system. The resonant grounding means thus substantially limit the ground fault current to zero to reduce mechanical stress. Another continuous operation of the device is allowed after the occurrence of phase to ground faults until an ordered stop operation is arranged.

7 shows an embodiment with a non-linear resistance arrester 18 connected between the neutral point 20 and ground and a ground transformer 30 connected to the network side of the device 14. The ground transformer 30 is a Y-Δ design with a neutral point of the Y connection connected to ground, and the Δ-winding is insulated. Ground transformers are commonly used in systems that are not grounded or have high impedance ground connections. Like system components, the ground transformer has no load and does not affect normal system operation. When unbalance occurs, the ground transformer provides a small impedance to the zero sequence network. In this way, a ground transformer is like a small inductance or a small resistive ground in terms of fault current level and transient overvoltage.

A ground transformer is a so-called zigzag transformer with a specific winding arrangement (see "Analysis of Faulted Power Systems" by Paul M. Anderson of Iowa State University Press / Ames, 1983).

Possible auxiliary power transformers can also be used for the ground.

8 shows an embodiment with a small ohm resistor 32 connected between the neutral point 20 of the device 14 and ground. The main advantage of the small resistive ground is its ability to limit transients and transient overvoltages. However, the current is higher in the case of one phase ground fault. The third harmonic current is also higher in unobstructed operation.

9 shows another embodiment of the device according to the invention in which the resistor 32 is replaced by a small inductance inductor 34 connected between the neutral point 20 and ground. Small inductance ground works in the same way as small ohm ground. At ohms the value of inductor 34 is less than that required for resonant ground in the description of FIG. 6.

As an alternative to the direct connection between the neutral point 20 and the ground of the resistor 32 or inductor 34, they may be connected between the primary side and the neutral point 20 and ground and the secondary side, as in the technique of FIG. It can be indirectly connected with the aid of a transformer comprising an inductor.

The embodiment in FIG. 10 includes two impedances 36 and 38 connected in series between the neutral point 20 of the device 14 and ground, the impedance 36 having a small impedance value and the impedance 38 being It has a high impedance value. Impedance 38 may be shorted by shorting device 40. In normal operation, the shorting device 40 is opened to minimize the third harmonic current. In the case of a ground fault, the shorting device 40 controls to short the impedance 38 and the potential of the neutral point 20 is small and the current passes through the ground relatively high.

In the case of an internal ground fault in the device 14, the impedance 38 is not shorted. As a result, the voltage is high at the neutral point 20 but the current to ground will be limited. High currents in this situation are preferable because they cause damage in this case.

Tuning in to the overvoltage protector 39 and the third harmonic, in order to cope with the problems arising from the third harmonic when the AC electrical device is directly connected to a three-phase power network, i.e. when the incremental transformer is not used between the device and the network. Grounding means in the form of suppressed filters 35, 37 can be used with reference to FIG. 11. The filter thus comprises a parallel resonant circuit composed of an inductor 35 and a capacitive reactance 37. The size of the filters 35 and 37 and its high voltage protector 39 are such that the parallel circuit can absorb third harmonics from the device during normal operation but can limit transients and transient overvoltages. In the case of a fault, the overvoltage protector 39 limits the fault voltage such that a fault current flows through the overvoltage protector 39 if the fault is large. In the case of a single-phase ground fault, the current is higher than in the case of a high resistance ground because the fundamental impedance is small.

An embodiment in FIG. 12 is shown and the grounding means comprises an untuned switchable third harmonic suppression filter connected in parallel with the overvoltage protector 40. The filter is realized in several different ways and FIG. 12 shows an embodiment comprising two inductors 42, 44 connected in series and a capacitor 46 connected in parallel to the series connected inductors 42, 44. do. Another shorting device 48 is connected across the inductor 44.

The shorting device 48 controls to change the characteristics of the filter by shorting the inductor 44 when a risk for a third harmonic resonance between the filter and the device 14 and the network 16 is detected. This is described in detail in Swedish patent application 9700347-9. In this way the third harmonic current is strongly limited to normal operation. Transients and transient overvoltages are limited in the manner described in connection with FIG. 11 and the current will be higher than for single phase ground faults.

FIG. 13 shows an embodiment in which the neutral point 20 of the device 14 is directly connected to ground 21. Direct grounding limits transients and transient overvoltages, but in the case of ground faults it causes high currents. The third harmonic current flow from the neutral point 20 of the device to ground is relatively higher than in normal operation.

Another alternative of the grounding means according to the invention comprises a circuit for providing a connection of the neutral point to ground having a desired impedance characteristic.

In FIG. 14 the Δ connection three-phase device 50 is shown directly in the distribution or transport network 16.

In this situation, a ground transformer as used in the embodiment shown in FIG. 7 may be connected on the network side of the device 50.

As in the embodiment of Fig. 7, the ground transformer is a Y-Δ connection transformer having a neutral point of Y connection ground, or a so-called zigzag ground transformer, that is, a Z-grounded Z-O connection transformer. The ground transformer will limit the transient overvoltage.

Possible auxiliary power transformers as in the embodiment of FIG. 7 can be used for this purpose.

Claims (34)

In a high voltage AC device directly connected to a power distribution or power transmission network 16 and comprising at least one insulated current transport conductor 4, A first layer 6 having semiconductor characteristics is provided around the conductor 4, a solid insulating layer 7 is provided around the first layer, and a second layer 8 having semiconductor characteristics is the insulation Are provided around the floor and the grounding means 18, 21, 22, 24, 26, 28, 30, 32, 34, 35, 36, 37, 38, 39, 40, 42, 44, 46, 48, 52 And at least one point of the winding is connected to ground. 2. The high voltage AC device of claim 1, wherein the potential of the first layer is equal to that of a conductor. 3. The high voltage AC device of claim 1 or 2, wherein the second layer is arranged to form a coincidence surface surrounding the conductor. 4. The high voltage AC device of claim 3, wherein the second layer is connected to a predetermined potential. 5. The high voltage AC device of claim 4, wherein the predetermined potential is a ground potential. 6. The high voltage AC device of claim 1, 2, 3, 4 or 5, wherein at least two adjacent layers have the same coefficient of thermal expansion. 7. The high voltage AC device of any of claims 1-6, wherein the current carrying conductor comprises a plurality of strands and only a few strands are not insulated from each other. 8. The high voltage AC device of any of claims 1 to 7, wherein each of said three layers is connected and fixed to an adjacent layer along the entire connecting surface. An AC device having a magnetic circuit for high voltage comprising a magnetic core and at least one winding, The winding is formed of a cable comprising one or more current carrying conductors, each conductor comprising a plurality of strands, an inner semiconductor layer provided around each conductor, an insulating layer of solid insulating material provided around the inner semiconductor layer, and the An external semiconductor layer provided around the insulating layer, wherein a grounding means is provided for connecting at least one point of the winding to ground. 10. The AC device of claim 9, wherein the cable comprises a metal shield and a sheath. The AC device according to claim 1, wherein the grounding means comprises means for directly grounding the winding. The AC device according to claim 1, wherein the grounding means comprises a small resistance grounding means for the winding. 13. AC according to claim 12, wherein the device has a Y-connected winding and the neutral point of the Y connected winding is available and the small resistance grounding means comprises a small resistor resistor connected between the neutral and ground. Device. 13. The apparatus according to claim 12, wherein the device has a Y connecting winding, the neutral point of the Y connecting winding is available, and the small resistance grounding means has a resistor connected to the secondary side of the transformer whose primary side is connected between the neutral point and ground. AC device comprising a. 11. An AC device according to any one of the preceding claims, wherein the grounding means comprises a small inductance grounding means for the windings. 16. An AC device according to claim 15, wherein the device has a Y connecting winding and the neutral point of the Y connecting winding is available and the small inductance grounding means comprises a small inductance inductor connected between the neutral point and ground. . 16. The apparatus of claim 15, wherein the device has a Y connecting winding and the neutral point of the Y connecting winding is available and the small inductance grounding means is connected to the secondary side of the transformer whose primary side is connected between the neutral point and ground. AC device. The AC device according to claim 1, wherein the grounding means comprises a high resistance grounding means for the winding. 19. An AC device according to claim 18, wherein the device has a Y connecting winding and the neutral point of the Y connecting winding is available and the high resistance grounding means comprises a high resistance resistor connected between the neutral and ground. . 19. The apparatus of claim 18, wherein the device has a Y connecting winding and a neutral point of the Y connecting winding is available and the high resistance grounding means comprises a resistor connected to the secondary side of the transformer whose primary side is connected between the neutral point and ground. AC device comprising a. 11. An AC device according to any one of the preceding claims, wherein the grounding means comprises a high inductance grounding means for the windings. 22. The AC device of claim 21 wherein the device has a Y connecting winding and the neutral point of the Y connecting winding is available and the high inductance grounding means comprises a high inductance inductor connected between the neutral point and ground. . 22. The apparatus of claim 21, wherein the device has a Y connecting winding and a neutral point of the Y connecting winding is available and the high inductance grounding means uses an inductor connected to the secondary side of the transformer whose primary side is connected between the neutral point and ground. AC device comprising a. The device according to claim 1, wherein the device is a Y connecting winding and the neutral point of the Y connecting winding is available and the grounding means is connected to the secondary side of the transformer whose primary side is connected between the neutral point and ground. And a connected reactor, said reactor having a property such that during a ground fault the capacitive current is neutralized by the same component of the induced current provided by the reactor. The AC device according to claim 1, wherein the grounding means comprises means for changing a connection impedance to ground in response to a ground fault. 11. AC device according to one of the preceding claims, wherein the grounding means comprises an active circuit. The AC device according to claim 1, wherein the grounding means comprises a Y-Δ grounding transformer connected to the network side of the device. The AC device according to claim 1, wherein the grounding means comprises a so-called zigzag grounding transformer connected to the network side of the device. 11. A device according to any one of the preceding claims, wherein the device has a Y connecting winding and the neutral point of the Y connecting winding is available and the grounding means comprises a suppression filter tuned to the nth harmonic. AC device. The apparatus of claim 1, wherein the apparatus has a Y connecting winding and the neutral point of the Y connecting winding is available and the grounding means comprises a switchable suppression filter which is not tuned to the nth harmonic. AC device, characterized in that. 31. The AC device according to claim 29 or 30, wherein the nth harmonic is a third harmonic. The device according to claim 1, wherein the device has a Y connecting winding and the neutral point of the Y connecting winding is available and the grounding means comprises an overvoltage protector connected between the neutral point and ground. AC device characterized. 32. The device of any of claims 18 to 31, wherein the device has a Y connecting winding and a neutral point of the Y connecting winding is available, and an overvoltage protector is connected between the neutral point and ground in parallel to the grounding means. AC device characterized. 34. A power distribution or transmission network comprising at least one device according to any of the preceding claims.